Aesthetics has a lot to do with it; as well as tradition.
Originally hooks for Atlantic salmon flies were blind, that is, they had no eyes. An attachment was formed by making a loop of gut at the end of the hook as the fly was being tied.
Later, a metal loop eye was developed that roughly resembled the gut eyes. When the gut eyes were knotted on to the leader, using a Turle knot or similar, the gut loop was positioned at a slight upward angle. I think the manufacturers just retained this look in the metal eyes.
The turned-up-loop-eye hook (TULE) has become the standard for dressing elegant steelhead and Atlantic salmon flies (except for the real purists, who still use blind hooks and gut loops).
For practical justification, The TULE hook is very easy to attached with a double Turle and, thus tied, fishes very nice and straight.
There is also an added safety factor in the looped eye, as it will never straighten out, as ringed eyed hooks may (rarely) do. Also, there's no way the leader can catch and fray in a partially opened eye.
How these hooks came to be japanned (black laquered) rather than bronzed, I don't know.
Years ago, when I started steelheading, I was desparate to get TULE hooks. They were really hard to come by; now excellent quality TULE hooks are readily available from a number of makers in all sorts of styles, wire thicknesses and lengths. Ain't life grand.
Hope this helps.